1. Field of the Invention
The invention relates to screening apparatus particularly of the vibrating type, driven by an input motor turning eccentric weights combined with sub-resonant tuned "drive" springs.
2. Description of the Prior Art
Screening devices heretofore have been designed to operate horizontally with a gyratory motion intended to distribute material being screened over the major portion of the screening surface and some such devices had bouncing balls, or pellets, that were provided for the purpose of preventing the screen from becoming plugged up by the material. These balls, or pellets, were caused to bounce up-and-down against the screen surface by the vibratory movement of the machine to dislodge any material clogging the screen. Such machines relied more or less upon a "single input" or "brute force" vibratory drive system in order to effect their purpose and therefore consumed an inordinate amount of power to operate and consequently were very expensive to use.
This sifter screen arrangement maintains a sliding relationship of the material over the screen deck whereas prior devices of the linear stroke type, used a pronounced "pitch-and-catch" type of stroke action imparted to the material as it moved over the sifter deck at typical stroke angles of 30.degree., or greater, from the horizontal. A common conveyor type screening action utilized a 45.degree. stroke angle from the horizontal.
To change the magnitude of the operating stroke of these prior machines, it was necessary to shut the machine down completely and then after the set-up of the eccentric weights to modify the stroke. However, the customers preferred a low power consuming apparatus with a flat stroke action for screening that was readily adjustable both with respect to the stroke and the frequency.
Prior U.S. Pat. No. 4,287,056 of Sept. 1, 1981, provided a Sifter Stroke Screen which had a vibratory flat stroke driven by a squirrel cage motor which afforded a readily adjustable stroke and frequency simply by varying the voltage to the motor. This system of adjusting the sifter operation was disclosed in my prior U.S. Pat. No. 3,251,457 of May 17, 1966. In U.S. Pat. No. 4,287,056 the apparatus included the drive motor with rotating eccentric weights at the inlet end of the sifter but the drive springs, and counterbalance were located at the outlet end of the sifter. Such arrangement was disadvantageous inasmuch as the overall length of the sifter apparatus was increased and resulted in the counterbalance apparatus projecting objectionably at the downstream end of the sifter.